Sputtering, alternatively called physical vapor deposition (PVD), has long been used in depositing metals and related materials in the fabrication of semiconductor integrated circuits. Use of sputtering has been extended to depositing metal layers onto the sidewalls of high aspect-ratio holes such as vias or other vertical interconnect structures, as well as in the manufacture of extreme ultraviolet (EUV) mask blanks. In the manufacture of EUV mask blanks minimization of particle generation is desired, because particles negatively impact the properties of the final product.
Plasma sputtering may be accomplished using either DC sputtering or RF sputtering. Plasma sputtering typically includes a magnetron positioned at the back of the sputtering target to project a magnetic field into the processing space to increase the density of the plasma and enhance the sputtering rate. Magnets used in the magnetron are typically closed loop for DC sputtering and open loop for RF sputtering.
In plasma enhanced substrate processing systems, such as physical vapor deposition (PVD) chambers, high power density PVD sputtering with high magnetic fields and high DC Power can produce high energy at a sputtering target, and cause a large rise in surface temperature of the sputtering target. The sputtering target is cooled by contacting a target backing plate with cooling fluid. However, it has been determined that such cooling may not be sufficient to capture and remove heat from the target. Remaining heat in the target can result in significant mechanical bowing due to thermal gradient in the sputter material and across backing plate. The mechanical bowing increases as larger size wafers are being processed. This additional size aggravates the tendency of the target to bow/deform under thermal, pressure and gravitational loads. The impacts of bowing may include mechanical stress induced in the target material that can lead to fracture, damage to the target, and changes in distance from a magnet assembly to the face of the target material that can cause changes in the plasma properties (e.g., moving the processing regime out of an optimal or desired processing condition which affects the ability to maintain plasma, sputter/deposition rate, and erosion of the target).
In addition, higher target temperature results in re-sputtering of target material, which causes particle generation and defects on other parts of the PVD chamber and the wafer being processed in the chamber. The thermal management of target cooling is important not only for target life but also for reducing particles and defects, which will improve process yield. There is need to provide apparatus and methods to efficiently cool PVD targets during physical vapor deposition processes.